Antisense suppression refers to the binding of an “antisense” strand of a nucleic acid to a gene or mRNA, thereby preventing expression of the gene or translation of the mRNA. Typically for antisense suppression, an expression cassette is designed to express an RNA molecule complementary to all or part of an mRNA encoding a target. Over-expression of the antisense RNA molecule may result in reduced expression of the native gene.
The polynucleotide for use in antisense suppression may correspond to all or part of the complement of the sequence encoding the target, all or part of the complement of the 5′ and/or 3′ untranslated region of the target transcript, and/or all or part of the complement of both the coding sequence and the untranslated regions of a transcript encoding the target. In addition, the antisense polynucleotide may be fully complementary (i.e., 100% identical to the complement of the target sequence) or partially complementary (i.e., less than 100% identical to the complement of the target sequence) to the target sequence. Antisense suppression may be used to inhibit the expression of multiple proteins in the same cell or organism, as described, for example, in U.S. Pat. No. 5,952,657. Furthermore, portions of the antisense nucleotides may be used to disrupt the expression of the target gene. Generally, sequences of at least 50, 100, 200, 300, 500, or 550 nucleotides may be used. Methods for using antisense suppression to inhibit the expression of endogenous genes in plants are described, for example, in Liu et al. (2002), Plant Physiol. 129:1732-1753 and U.S. Pat. Nos. 5,759,829 and 5,952,657, each of which is herein incorporated by reference. Efficiency of antisense suppression may be increased by including a poly-dT region in the expression cassette at a position 3′ to the antisense sequence and 5′ of the polyadenylation signal. See, U.S. Patent Publication No. 20020058815, herein incorporated by reference.
RNA interference refers to the process of sequence-specific post-transcriptional gene silencing in animals mediated by short interfering RNAs (siRNAs) (Fire et al., 1998, Nature 391, 806; Hamilton et al., 1999, Science 286, 950-951). The corresponding process in plants is commonly referred to as post-transcriptional gene silencing or RNA silencing, and is also referred to as quelling in fungi. The process of post-transcriptional gene silencing is thought to be an evolutionarily conserved cellular defense mechanism used to prevent the expression of foreign genes and is commonly shared by diverse flora and phyla (Fire et al., 1999, Trends Genet. 15, 358). Such protection from foreign gene expression may have evolved in response to the expression of double-stranded RNAs (dsRNAs) derived from viral infection or from the random integration of transposon elements into a host genome via a cellular response that specifically destroys homologous single-stranded RNA or viral genomic RNA. The presence of dsRNA in cells triggers the RNAi response through a mechanism that has yet to be fully characterized. This mechanism appears to be different from the interferon response that results from dsRNA-mediated activation of protein kinase PKR and 2′,5′-oligoadenylate synthetase resulting in non-specific cleavage of mRNA by ribonuclease L.
The presence of long dsRNAs in cells stimulates the activity of a ribonuclease III enzyme referred to as dicer. Dicer is involved in the processing of the dsRNA into short pieces of dsRNA known as short interfering RNAs (siRNAs) (Hamilton et al., supra; Berstein et al., 2001, Nature 409, 363). Short interfering RNAs derived from dicer activity are typically about 21 to about 23 nucleotides in length and comprise about 19 base pair duplexes (Hamilton et al., supra; Elbashir et al., 2001, Genes Dev. 15, 188). Dicer has also been implicated in the excision of 21- and 22-nucleotide small temporal RNAs (stRNAs) from precursor RNA of conserved structure that are implicated in translational control (Hutvagner et al., 2001, Science 293, 834). The RNAi response also features an endonuclease complex, commonly referred to as an RNA-induced silencing complex (RISC), which mediates cleavage of single-stranded RNA having sequence complementary to the antisense strand of the siRNA duplex. Cleavage of the target RNA takes place in the middle of the region complementary to the antisense strand of the siRNA duplex (Elbashir et al., 2001, Genes Dev. 15, 188).
RNAi has been studied in a variety of systems. Fire et al., 1998, Nature 391, 806, were the first to observe RNAi in C. elegans. Bahramian and Zarbl, 1999, Molecular and Cellular Biology 19, 274-283, and Wianny and Goetz, 1999, Nature Cell Biol. 2, 70, describe RNAi mediated by dsRNA in mammalian systems. Hammond et al., 2000, Nature 404, 293, describe RNAi in Drosophila cells transfected with dsRNA. Elbashir et al., 2001, Nature 411, 494, describe RNAi induced by introduction of duplexes of synthetic 21-nucleotide RNAs in cultured mammalian cells including human embryonic kidney and HeLa cells. Methods for using dsRNA interference to inhibit the expression of endogenous plant genes are described in Waterhouse et al. (1998), Proc. Natl. Acad. Sci. USA 95:13959-13965, Liu et al. (2002), Plant Physiol. 129:1732-1753, and WO 99/59029, WO 99/53050, WO 99/61631, and WO 00/59035.
Additional RNAi methods relating to the inhibition of the expression of one or more targets obtained by hairpin RNA (hpRNA) interference or intron-containing hairpin RNA (ihpRNA) interference have been described. These methods are highly efficient at inhibiting the expression of endogenous genes. See, Waterhouse and Helliwell (2003) Nat. Rev. Genet. 5:29-38 and the references cited therein.
For hpRNA interference, the expression cassette is designed to express an RNA molecule that hybridizes with itself to form a hairpin structure that comprises a single-stranded loop region and a base-paired stem. The base-paired stem region comprises a sense sequence corresponding to all or part of the endogenous messenger RNA encoding the gene whose expression is to be inhibited, and an antisense sequence that is fully or partially complementary to the sense sequence. Thus, the base-paired stem region of the molecule generally determines the specificity of the RNA interference. hpRNA molecules are highly efficient at inhibiting the expression of endogenous genes, and the RNA interference they induce is inherited by subsequent generations. See, for example, Chuang and Meyerowitz (2000) Proc. Natl. Acad. Sci. USA 97:5985-5990; Stoutjesdijk et al. (2002), Plant Physiol. 129:1723-1731; and Waterhouse and Helliwell (2003) Nat. Rev. Genet. 5:29-38. Methods for using hpRNA interference to inhibit or silence the expression of genes are described, for example, in Chuang and Meyerowitz (2000) Proc. Natl. Acad. Sci. USA 97:5985-5990; Stoutjesdijk et al. (2002), Plant Physiol. 129:1723-1731; Waterhouse and Helliwell (2003) Nat. Rev. Genet. 5:29-38; Pandolfini et al., BMC Biotechnology 3:7, and U.S. Patent Publication No. 20030175965. A transient assay for the efficiency of hpRNA constructs to silence gene expression in vivo has been described by Panstruga et al. (2003), Mol. Biol. Rep. 30:135-150.
For ihpRNA, the interfering molecules have the same general structure as for hpRNA, but the RNA molecule additionally comprises an intron that is capable of being spliced in the cell in which the ihpRNA is expressed. The use of an intron minimizes the size of the loop in the hairpin RNA molecule following splicing, which increases the efficiency of interference. See, for example, Smith et al. (2000), Nature 507:319-320. In fact, Smith et al. show 100% suppression of endogenous gene expression using ihpRNA-mediated interference. Methods for using ihpRNA interference to inhibit the expression of genes are described, for example, in Smith et al. (2000), Nature 507:319-320; Wesley et al. (2001), Plant J 27:581-590; Wang and Waterhouse (2001) Curr. Opin. Plant Biol. 5:156-150; Waterhouse and Helliwell (2003) Nat. Rev. Genet. 5:29-38; Helliwell and Waterhouse (2003) Methods 30:289-295, and U.S. Patent Publication No. 20030180955.
Others have reported on various RNAi and gene-silencing systems. For example, Parrish et al., 2000, Molecular Cell 6, 1077-1087, describe specific chemically modified siRNA constructs targeting the unc-22 gene of C. elegans. Grossniklaus, International PCT Publication No. WO 01/38551, describes certain methods for regulating polycomb gene expression in plants using certain dsRNAs. Churikov et al., International PCT Publication No. WO 01/42443, describe certain methods for modifying genetic characteristics of an organism using certain dsRNAs. Cogoni et al., International PCT Publication No. WO 01/53475, describe certain methods for isolating a Neurospora-silencing gene and uses thereof. Reed et al., International PCT Publication No. WO 01/68836, describe certain methods for gene silencing in plants. Honer et al., International PCT Publication No. WO 01/0944, describe certain methods of drug screening using transgenic nematodes as Parkinson's disease models using certain dsRNAs. Deak et al., International PCT Publication No. WO 01/72774, describe certain Drosophila-derived gene products that may be related to RNAi in Drosophila. Arndt et al., International PCT Publication No. WO 01/92513 describe certain methods for mediating gene suppression by using factors that enhance RNAi. Tuschl et al., International PCT Publication No. WO 02/44321, describe certain synthetic siRNA constructs. Pachuk et al, International PCT Publication No. WO 00/63364, and Satishchandran et al., International PCT Publication No. WO 01/04313, describe certain methods and compositions for inhibiting the function of certain polynucleotide sequences using certain dsRNAs. Echeverri et al., International PCT Publication No. WO 02/38805, describe certain C. elegans genes identified via RNAi. Kreutzer et al., International PCT Publications Nos. WO 02/055692, WO 02/055693, and EP 1144623 B1 describe certain methods for inhibiting gene expression using RNAi. Graham et al., International PCT Publications Nos. WO 99/49029 and WO 01/70949, and AU 4037501 describe certain vector expressed siRNA molecules. Fire et al., U.S. Pat. No. 6,506,559, describe certain methods for inhibiting gene expression in vitro using certain long dsRNA (greater than 25 nucleotides) constructs that mediate RNAi.
Although much work has been done in the area of gene silencing using RNAi and antisense technologies, improvements that allow increased modulation of gene expression over RNAi or antisense technology would be an improvement in the art.